39.6 Greville abscissae

The Greville abscissae are defined to be the mean location of k-1
consecutive knots in the knot vector for each basis spline function of order
k. Note that the first and last knots in the knot vector are excluded
when applying this definition; consequently there are
gsl_bspline_ncoeffs Greville abscissa. They are often used in B-spline
collocation applications and may also be called Marsden-Schoenberg points.

The above definition is undefined for k=1. The implementation chooses
to return interval midpoints in the degenerate k=1 case.